It is really hard to share your child with your ex and their new partner. Some parents revel in having a couple of days off a week from being in full-time parent mode, but many would much rather have their kid for every single second of every single day. Having split custody makes holidays so complicated. You have your kids on Christmas Eve, but your ex has them on Christmas. That can be a really upsetting situation for you and the kids, especially if they don't necessarily like being around your ex and their new flame. But if you're ordered by the court to have your kids for a specific amount of time, with the other time going to your ex, there really isn't anything you can do about it unless you want to go back to court.
It's also not easy to be on the other side of this situation. If you're the stepparent of a kid who spends half of their time with you, you might be put in the awkward position of trying to accommodate their other parent, who wishes they could be with their kid all the time. They want to infringe on your partner's time with their kid, and they want you to help them do it.
u/SignificantRent5758, the stepmom in this story, has a pretty civil relationship with the mother of her stepson, but she's being put in an awkward position. The mom of her stepson wants to sleep over the night of his birthday, and the stepmom feels uncomfortable letting her do so.